Voltage sensitive dyes (VSD), also known as potentiometric dyes or voltage sensitive chromophores, have been used to determine membrane potentials for biological cells. Such dyes have been designed to become embedded within a cell membrane orthogonal to the membrane surface where they are then exposed to the membrane electric field. U.S. Patent Application Publication 2010/0074847 to Madden et al., describes the use of chromophore probes for optical imaging of biological materials. The use of voltage sensitive dyes, is also discussed by L. Loew in an article titled “Potentiometric dyes: Imaging electrical activity in cell membranes” (Pure & Appl. Chem., Vol. 68, pp. 1405-1409, 1996). The use is very similar to that described U.S. Patent Application Publication 2010/0074847.
The absorbance or fluorescence emission of a VSD can be measured for the membrane in both the polarized state and un-polarized state. The shift in the resulting spectrum gives the electrical potential difference between the two states.
Two types of VSDs, which vary in their response mechanism, are known. What are considered “slow” VSDs partition within the cell, and their fluorescence intensity is a Nernstian-concentration-dependent response. What are considered “fast” VSD's do not depend on partitioning and instead respond to the electric field directly via the Stark effect.
Experimental strategies for measuring electric field strength (intensity) at charged solid surfaces using a fluorescent dye as a probe particularly in bulk behavior are described by J. Pope et al. in an article entitled “Measurement of electric fields at rough metal surfaces by electrochromism of fluorescent probe molecules embedded in self-assembled monolayers” (J. Am. Chem. Soc., Vol. 114, pp. 10085-10086, 1992), as well as by J. Pope et al. in an article entitled “Measurements of the potential dependence of electric field magnitudes at an electrode using fluorescent probes in a self-assembled monolayer” (J. Electroanalytical Chem., Vol. 498, pp. 75-86, 2001).
U.S. Pat. No. 5,156,918 (Marks et al.) describes the use of a poly(phenylene ether) to which a pyridine-terminated chromophore is attached and the resulting nonlinear optical material can be covalently attached to solid surfaces as a monolayer for various purposes.
Surface potential is a key parameter in colloidal and biological sciences governing interactions between materials such as the attractive or repulsive interaction between materials. Thus, adhesion forces between particles or polymers and a surface that affects particle patterning are directly influenced by surface potentials. Such particles and polymers cover a wide range of materials ranging from biological to inorganic materials. Currently there are two prominent methods to determine surface potentials. However, each of these methods create at least one problem. For example, the method of using streaming potentials described by H. Xie et al. in the article “Zeta potential of ion-conductive membranes by streaming current measurements” (Langmuir, Vol. 27, pp. 4721-4727, 2011) requires expensive equipment and the method is damaging to tested samples. Moreover, the use of atomic force microscopy as described by S. Li et al. in the article “Excluding contact electrification in surface potential measurement using Kelvin probe force microscopy” (ACS Nano, Vol. 10, pp. 2528-2535, 2016) is restricted to very small areas and requires long scanning times.
There remains a need for less burdensome and more versatile methods for determining solid-liquid interface electrical field intensities, and for determining the characteristics of liquids.